When does vote by mail cost less?

I was fascinated by the story that Charles posted out of Napa Valley.

I just got back from DC where I testified in front of Mary Cheh’s committee of the DC Council on the costs of conducting an upcoming special election.  Rokey Suleman estimated that a vbm election in the District would cost nearly double (1.6 million vs. $800,000) of a traditional precinct place election.

Is this because Napa already mails out so many absentee ballots?  Because they already have to mail out sample ballots?  Keep in mind that however Napa gears up for a precinct place election, the costs will not be comparable to the 2010 election because turnout will likely be so much lower.